This research aims at an investigation of the thoracic chemoreceptors (aortic bodies) of dogs. It is particularly directed toward those in group 4 which lie between the aorta and pulmonary artery. These have a blood supply derived from a small artery which originates from the common left coronary artery or one of its major branches. Serotonin injected through an exploring intracoronary catheter will be used to locate the site which promotes the maximum reflex pressor response so that the small vessel can be identified and isolated. The left coronary artery will be cannulated with a new silicone rubber tipped cannula which does not require dissection. Coronary flow will be measured and the left coronary artery and small branch will be perfused at different pressures and with bloods of different PO2 PCO2 and pH. We will investigate the role of the small vessel in a reflex depressor response which follows elevation of perfusion pressure in the left coronary artery to determine whether chemoreceptor or other mechanisms are involved. We will study the possible coronary resistance changes induced by stimulation of these chemoreceptors, and the role of the small vessel in reflex changes in peripheral resistance after coronary occlusion. Possible long term effects following occlusion of the small vessel will be investigated in trained dogs.